For several years, makers of electronic and electromechanical systems have known that IC stacking methods and stacked devices can sometimes allow more components to be mounted in a given area of a substrate. For example, U.S. Pat. No. 5,612,570 (filed Apr. 13, 1995 by Eide et al.) teaches a configuration for stacking one chip into each of several frames, and then stacking the frames. Signal routing between the chip leads and the frames is provided by traces routed through the frames.
Although many known stacking methods are capable of providing the desired PCB population density, a first problem not yet adequately addressed is that identical (or very similar) chips in a vertically aligned group have required at least as many interfaces containing routing (i.e. horizontally offset vertical conductors) as chip layers. Each alternation between a chip layer and an interface structure adds to the cost of the handling equipment required to assemble a stack.
U.S. Pat. No. 4,956,694 (filed Nov. 4, 1988 by Eide et al.) teaches a configuration for stacking slightly differing LCC chips onto a small substrate that can then be mounted sideways on a larger substrate. This stacked device relies on these slight differences between the dies to function, because exactly identical IC dies connected exactly in parallel are unable to perform logic functions individually. A second problem not yet adequately addressed in the art is that stacked device configurations of this type require that the dies be fabricated using different masks, and are subsequently maintained in distinct inventories. A need thus exists for stacked devices that can be made with dies that are fabricated identically and then made into distinct chips at a later step of manufacture.
A third problem that exists in the art is very long, high capacitance conduits typically provided between internal circuit elements on different dies. Although some prior stacking configurations incidentally reduce the length and capacitance of such conduits as compared to electrical paths comprising internal traces of a substrate, all known configurations suffer from either difficulty of manufacture or relatively poor performance.